Dissertations / Theses on the topic 'Snap-Through'

Snap-through is a striking instability in which an elastic object rapidly jumps from one state to another. It is seen in the leaves of the Venus flytrap plant and umbrellas flipping on a windy day among many other examples. Similar structures that snap-through are used to generate fast motions in soft robotics, switches in micro-scale electronics and artificial heart valves. Despite the ubiquity of snap-through in nature and engineering, its dynamics is usually only understood qualitatively. In this thesis we develop analytical understanding of this dynamics, focussing on how the mathematical structure underlying the snap-through transition controls the timescale of instability. We begin by considering the dynamics of 'pull-in' instabilities in microelectromechanical systems (MEMS) - a type of snap-through caused by electrostatic forces in which the motions are dominated by fluid damping. Using a lumped-parameter model, we show that the observed time delay near the pull-in transition is a type of critical slowing down - a so-called 'bottleneck' due to the 'ghost' of a saddle-node bifurcation. We obtain a scaling law describing this slowing down, and, in the process, unify a large range of experiments and simulations that exhibit delay phenomena during pull-in. We also investigate the pull-in dynamics of MEMS microbeams, extending the lumped-parameter approach to incorporate the details of the beam geometry. This provides a model system in which to understand snap-through of a continuous elastic structure due to external loading. We develop a perturbation method that systematically exploits the proximity to pull-in to reduce the governing equations to a simpler evolution equation, with a structure that highlights the saddle-node bifurcation. This allows us to analyse the bottleneck dynamics in detail, which we compare with previous experimental and numerical data. The remainder of the thesis is concerned with the dynamics of snap-through in macroscopic systems. In particular, we explore the extent to which dissipation is required to explain anomalously slow snap-through. Considering an elastic arch as an archetype of a snapping system, we use the perturbation method developed earlier to show that two bottleneck regimes are possible, depending delicately on the relative importance of external damping. In particular, we show that critical slowing down occurs even in the absence of damping, leading to a new scaling law for the snap-through time that is confirmed by elastica simulations and experiments. In many real systems material viscoelasticity is present to some degree. Finally, we examine how this influences the snap-through dynamics of a simple truss-like structure. We present a regime diagram that characterises when the timescale of snap-through is controlled by viscous, elastic or viscoelastic effects.

The motion of a shallow arch subjected to random loading is investigated. It is expected that the arch will vibrate about its initial stable configuration under low intensity loading, but will snap back and forth about its horizontal axis under high intensity loading. The parameter of interest is the time of first snap-through of the shallow arch under random load. This is defined as the time taken for the arch to snap to the other side of the horizontal axis. The statistics of the time to first snap-through, such as the mean time to failure as well as its probability distribution. are determined. Most of the work treats one response mode. In the first part of the study, the critical random loading for dynamic snap-through of the shallow arch was investigated using the method of computer simulation. The random excitation was assumed to be a stationary white-noise process. The primary object was to determine the critical power spectral density parameter of the random excitations. The vanishing or diminishing of the average frequency of snap-through was used to estimate this parameter. An exact value of the critical random loading parameter could not be obtained using this criterion since it was based on numerical integration of the non-linear equation of motion and computer simulation which is expensive and time-consuming. However, the critical value or range of critical values of intensity of random excitations could be estimated with a reasonable degree of accuracy. The second part of the study dealt with the first-passage problem. The exact solution of the first-passage problem is available for only a limited class of problems. In this study, the solution was obtained using numerical approximation techniques and computer simulation. For an oscillator subjected to white noise, the displacement and velocity process are governed by the Fokker-Planck-Kolmogorov (FPK) equation. The finite difference method was used to solve the derived FPK equation of the energy envelope of the equation of motion of the arch subjected to white-noise excitation. Solutions were obtained in terms of the mean time to failure, and the probability distribution function of the time to failure. A computer program for the Monte Carlo simulation of the response of the arch subjected to random loads was also developed. A large number of records of the random excitations were simulated and these were used as input in the numerical integration of the equation of motion. The Runge-Kutta method was used to obtain the time history of the displacement response, and the time at which the response exceeded the critical threshold was recorded. Statistics of the time to first snap-through were obtained and these were then used to select an empirical distribution model for the first-passage time. The results of the approximate analysis were compared with those from the simulation. Results of both methods were in close agreement. The effect of including more than one mode in the equation of motion was also studied. Multi-mode approximations of up to four modes were considered in the analysis. It was found that the results of the multi-mode approximations are significantly different from the one-mode approximation. The effect of nonstationary random excitation on the time to first snap-through was also investigated using computer simulation.
Ph. D.

Dans cette thèse à l’interface entre élasticité et capillarité, nous présentons tout d’abord une instabilité élastique, le claquage, revisitée dans un contexte élastocapillaire. En déposant une goutte d’eau sous une lamelle flambée en position basse, nous parvenons à déclencher une instabilité de claquage à contresens de la gravité. Cette démonstration de la prédominance des effets capillaires à petite échelle s’accompagne d’une étude des positions d’équilibre et de la stabilité de systèmes goutte-lamelle. Nous démontrons l’influence importante de la taille et de la position de la goutte le long de la lamelle, puis étendons notre étude au cas de bulles ou de gouttes condensées à partir de vapeur d’eau. Enfin, nous nous intéressons à l’aspect dynamique de l’instabilité, qui est dictée principalement par l’élasticité, y compris dans le cas élastocapillaire.Nous mettons ensuite en évidence un phénomène surprenant : la dynamique de descente d’une goutte d’eau sur un élastomère silicone présente deux régimes successifs, caractérisés par deux vitesses différentes. Nous montrons que les chaînes libres non réticulées présentes dans l’élastomère sont à l’origine de cette dynamique inattendue. La goutte est progressivement recouverte par des chaînes de silicone, et sa vitesse change brutalement lorsqu’une concentration surfacique critique est atteinte, ce qui se traduit par une transition brutale de tension de surface. Nous nous intéressons aux vitesses de gouttes dans les deux régimes ainsi qu’aux échelles de temps mises en jeu lors de l’extraction de chaînes non réticulées, et montrons que l’extraction de ces chaînes se produit au niveau de la ligne triple
This thesis focuses on interactions between liquids and elastic solids. We first revisit the snap-through instability from an elastocapillary point of view, showing that capillary forces are able to counterbalance gravity by inducing snap-through with a droplet deposited below a downward buckled elastic strip clamped at both ends. Equilibrium, stability, and dynamics of drop-strip systems are investigated, demonstrating the influence of droplet size and droplet position along the buckled strip, and showing that capillarity is driving the system toward instability but elasticity is ruling the subsequent dynamics. Spin-off versions of the experiment are also designed, including a humidity-controlled mechanical switch and upscaled experiments using soap bubbles.We then focus on interactions between silicone elastomers and aqueous droplets to understand the mechanisms underlying an unexpected two-regime droplet dynamics observed on vertical silicone elastomer plates. After demonstrating that this two-regime dynamics is due to the presence of uncrosslinked oligomers in the elastomer, we show that the speed transition coincides with a surface tension transition. A quantitative study of the droplets speeds in the two regimes is performed, and the timescale needed for uncrosslinked oligomers to cover the water-air interface is investigated both for sessile and moving droplets. We eventually show that uncrosslinked chains are extracted from the elastomer at the water - air - silicone elastomer triple line, and demonstrate that extraction occurs in various setups such as partially immersed silicone elastomer plates or air bubbles sliding up PDMS planes immersed in a water bath

The Selection during Niche AdaPtation (SNAP) hypothesis aims to explain how the gene order in bacterial chromosomes can change as the result of bacteria adapting to a new environment. It starts with a duplication of a chromosomal segment that includes some genes providing a fitness advantage. The duplication of these genes is preserved by positive selection. However, the rest of the duplicated segment accumulates mutations, including deletions. This results in a rearranged gene order. In this work, we develop a method to identify SNAP in bacterial chromosomes. The method was tested in Salmonella and Bartonella genomes. First, each gene was assigned an orthologous group (OG). For each genus, single-copy panorthologs (SCPos), the OGs that were present in most of the genomes as one copy, were targeted. If these SCPos were present twice or more in a genome, they were used to build duplicated regions within said genome. The resulting regions were visualized and their possible compatibility with the SNAP hypothesis was discussed. Even though the method proved to be effective on Bartonella genomes, it was less efficient on Salmonella. In addition, no strong evidence of SNAP was detected in Salmonella genomes.

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
A formação de emulsões está presente em diversas atividades industriais, incluindo o setor petrolífero. A análise de quebra de gota em capilares com garganta remete ao estudo da formação de emulsões e suas características em meios porosos. Esta dissertação apresenta um modelo numérico baseado nas equações de conservação, e na teoria da lubrificação. O modelo foi desenvolvido considerando as duas fases como fluidos Newtonianos, e o sistema de equações diferencias resultante foi resolvido pelo método de diferenças finitas centrais, implementado em ambiente MATLAB®. Determina-se o campo de pressão dos fluidos, bem como sua interface ao longo do tempo. A análise da influência de parâmetros, tanto da geometria do capilar quanto das propriedades dos fluidos, indicou comportamentos adequados em comparação com os dados disponíveis na literatura. Observou-se que a viscosidade do fluido da fase contínua possui maior influência no tempo de formação de gota, do que a viscosidade do fluido da fase dispersa. O tempo de quebra de gota da fase dispersa varia linearmente com a viscosidade da fase contínua. Observa-se que o raio da garganta do capilar influencia diretamente no tempo de quebra de gota, onde para raios menores, o tempo de formação de gota será menor do que para um raio de garganta maior, devido a um gradiente de pressão capilar mais elevado.
The snap off of emulsion drops appears in several industrial activities, including the oil industry. The analysis snap off in a constricted capillary is related to the study of emulsion formation and its characteristics in a porous media. This thesis presents a numerical model based on the conservation equations and lubrification theory developed for Newtonians fluids. The resulting system of differential equations was solved by finite difference and the code implemented in MATLB®. The analysis of the effect of different parameters, such as the geometry of the capillary and the liquid proprieties, indicated appropriate behaviors in comparison to data in the literature. It was observed that the viscosity of the continuous phase fluid has a stronger influence in the snap off time than the viscosity of the dispersed phase fluid. The snap off time rises with the viscosity of the continuous phase fluid. It was observed that the variation of the radius of the neck directly influences the snap off time, where smaller throat radius leads to shorter snap-off time, due to stronger capillary pressure gradient.

The postbuckling load carrying capacity of composite plates offers immense potential to their applications for loads exceeding their primary buckling load. However, such an efficient and economical usage of these plates can be reliable only if the nonlinear postbuckling behavior of these plates, which includes a good understanding of secondary buckling, is understood thoroughly. The present investigation is an attempt to understand secondary buckling of almost square composite clamped-simply supported plates, both unstiffened as well as stiffened, in some detail. With the help of the finite element method, a large number of numerical studies have been conducted to understand the secondary buckling characteristics. The sensitivity of these characteristics to variations in boundary conditions, lamination sequence, imperfections, and stiffener geometry has been considered. It has been found that the occurrence of secondary buckling in clamped-simply supported plates under uniform end shortening critically depends on the intensity of restrictions imposed on the inplane normal displacements along the unloaded simply supported edges of the plate. These restrictions could be due to the actual boundary conditions at these edges, or due to the presence of stiffeners along these edges. It has also been found that the presence of imperfections significantly delays the event of secondary buckling. Finally, it has been found that changes in lamination sequence of the plate alter its secondary buckling characteristics in ways that are, in general, quantitative in nature. The numerical investigations were followed by a limited number of experiments involving the testing of unstiffened as well as stiffened composite plates with the intent of augmenting the confidence in the numerical predictions made. Three different lamination sequences were considered during the testing phase of this investigation. It was found that the agreement between experimental data and numerical predictions was quite good. The occurrence of secondary buckling followed the predictions closely.
Ph. D.

As a new concept for morphing structures, the use of piezoelectric actuators to effect snap-through behavior of simple unsymmetric cross-ply composite laminates is examined. Many unsymmetric laminates have more than one stable room-temperature shape and can be snapped through from one stable shape to another. In this new concept for morphing structures, one or more piezoelectric actuators are bonded to unsymmetric laminates, and are then used to snap the laminate from one shape to another. The actuator would be used to change shape, but would not be required to maintain the shape. Using the Rayleigh-Ritz technique, several models are developed to predict the interaction between the base laminate and the actuator. In particular, the voltage (applied to the actuator) needed to snap the laminate is predicted. The NASA-LaRC Macro-Fiber Composite&174; (MFC&174;) actuator is chosen as the actuator of choice for this work. A laminate is manufactured, an actuator is bonded to the laminate, and experiments are performed. Since the agreement between the initial models and experimental results was not good, the models were revised. Good agreement between the predictions of the revised model and experiment is reached. Suggestions for future research directions are presented.
Ph. D.

General unsymmetric laminates exhibit large natural curvatures at room temperature. Additionally, inherent to most unsymmetric laminates is the presence of two stable configurations. Multiple configurations and stability issues arise because of the geometric nonlinearities associated with the large curvatures. The laminate can be changed from one stable configuration to the other by a simple snap-through action. This situation offers the opportunity to use shape memory alloys (SMA) attached to the laminate to generate the snap-through forces and change the shape of the laminate on command. Presented is a model which can predict SMA-induced deformations in general unsymmetric laminates and, particularly, the occurrence of the snap through. First, a methodology is developed to predict the deformations of flat general unsymmetric epoxy-matrix composite laminates as they are cooled from their elevated cure temperature. Approximations to the strain fields are used in the expression for the total potential energy, and the Rayleigh-Ritz approach is used to study equilibrium. To further study the laminate deformations, finite-element analyses are performed. Experimental results are presented which confirm the predictions of the developed theory and the finite-element analyses regarding the existence of multiple solutions and the magnitude of the deformations. Results are compared with those of several other investigators. Next, the deformation behavior of general unsymmetric laminates subjected to applied forces is studied. The principle of virtual work is used to derive the equilibrium equations relating the laminate deformations to the applied forces. By solving the equilibrium equations as a function of the force level, relations between the laminate deformations and the applied force are derived, and the force level at which the laminate changes shape is determined. Finally, an existing SMA constitutive model is implemented into the developed theory to predict the deformations of simple structures to SMA-induced forces. Experiments on a narrow aluminium plate with an externally attached SMA actuator are conducted. The experimental results show good agreement with the predictions from the developed theory. Next, the deformation behavior of general unsymmetric laminates subjected to SMA actuators is predicted using the developed theory. Experiments using SMA actuators to generate the snap through of nsymmetric laminates are conducted. Good correlation with the developed theory is obtained.
Ph. D.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Alves Godoy, Willians Roberto, análise de um oscilador não linear acoplado a um absorvedor tipo snap through truss (stta). Bauru: Faculdade de Engenharia Mecânica, Universidade Estadual Paulista Julio de Mesquita Filho, 2011, 102 pp., Dissertação (Mestrado). Recentemente os estudos sobre os absorvedores de energia vêm ganhando importância e destaque, principalmente na indústria automobilística e aeronáutica. Esse tipo de absorvedor permite fazer o controle passivo de um sistema vibratório. Na busca por melhores resultados, verificou-se a ansiedade da utilização de absorvedores não lineares, já que estes demonstraram poder receber energia além de uma pequena faixa de freqüencia de vibração do oscilador. Neste trabalho, considera-se o sistema vibratório constituído de um controlador passivo, o absorvedor tipo snap-through truss (STTA) acoplado a um oscilador submetido à excitação de um motor elétrico com uma excentricidade e potência limitada, caracterizando um oscilador não ideal (NIO). Busca-se utilizar o absorvedor STTA como um absorvedor não linear, aproveitando sua não linearidade geométrica, e estabelecer condições para que as amplitudes de movimento do sistema principal se tornem menores em determinadas condições. O principal objetivo nesse trabalho é obter as condições em que as amplitudes de movimento do NIO são menores durante a passagem pela ressonância, atenuando o efeito Sommerfeld. Os resultados obtidos nessa situação mostraram que o STTA é eficiente na redução das amplitudes de movimento do NIO antes e dentro da região de ressonâcia. Em contrapartida, ao sair da ressonância ficou evidente que o absorvedor deve ser retirado do sistema por implicar grandes amplitudes ao NIO. Além disso, um breve estudo da atuação do STTA sobre um sistema...
In recent years studies on energy absorver in order to reduce some systems vibration amplitudes has been gaining importance and prominence. This type of absorber allows vibrating system passive control. In order to better results, it was verified the need to use non-lienar absorbers as these have been shown to receive power more than a small band of vibration frequency of the oscillator. This work considers the vibrating system that consists of a snap-through truss absorber (STTA) coupled to an oscillator under excitation of an electric motor with an eccentricity and limited power, characterizing a non-ideal oscillator (NIO). it is aimed to use the absorver STTA as non-linear absorber and establish the conditions for the motion amplitudes become smaller under certain conditions the mian system. Therefore, this work aims to determine the conditions in which the main system, while it passes through resonance, has attenuated its amplitude of moviment and do not waste energy working in this condition. The main objective of this work is to obtain the conditions under which amplitudes motion of NIO are smaller in the passage through resonance, reducing the Sommerfeld effect. The results obtained in this situation showed that the STTA is effectie to reduce amplitudes motion of NIO before and within region of resonance. In contrast, out of resonance, it was evident that absorber must be removed from the system by imply large amplitudes to the NIO. In addition, a brief study of the performance of STTA on an ideal system was performed. In this situation, the STTA showed significant results in the attenuation of amplitudes motion of the oscillator. Therefore, this work shows that the use of snap-through truss absorber is a good alternative among passive abserbers in attenuation of Sommerfeld effect in non-ideal systems

Orientador: José Manoel Balthazar
Banca: Angelo Marcelo Tusset
Banca: Bento Rodrigues de Pontes Junior
Resumo: Alves Godoy, Willians Roberto, análise de um oscilador não linear acoplado a um absorvedor tipo "snap through truss" (stta). Bauru: Faculdade de Engenharia Mecânica, Universidade Estadual Paulista "Julio de Mesquita Filho", 2011, 102 pp., Dissertação (Mestrado). Recentemente os estudos sobre os absorvedores de energia vêm ganhando importância e destaque, principalmente na indústria automobilística e aeronáutica. Esse tipo de absorvedor permite fazer o controle passivo de um sistema vibratório. Na busca por melhores resultados, verificou-se a ansiedade da utilização de absorvedores não lineares, já que estes demonstraram poder receber energia além de uma pequena faixa de freqüencia de vibração do oscilador. Neste trabalho, considera-se o sistema vibratório constituído de um controlador passivo, o absorvedor tipo "snap-through truss" (STTA) acoplado a um oscilador submetido à excitação de um motor elétrico com uma excentricidade e potência limitada, caracterizando um oscilador não ideal (NIO). Busca-se utilizar o absorvedor STTA como um absorvedor não linear, aproveitando sua não linearidade geométrica, e estabelecer condições para que as amplitudes de movimento do sistema principal se tornem menores em determinadas condições. O principal objetivo nesse trabalho é obter as condições em que as amplitudes de movimento do NIO são menores durante a passagem pela ressonância, atenuando o efeito Sommerfeld. Os resultados obtidos nessa situação mostraram que o STTA é eficiente na redução das amplitudes de movimento do NIO antes e dentro da região de ressonâcia. Em contrapartida, ao sair da ressonância ficou evidente que o absorvedor deve ser retirado do sistema por implicar grandes amplitudes ao NIO. Além disso, um breve estudo da atuação do STTA sobre um sistema... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: In recent years studies on energy absorver in order to reduce some systems vibration amplitudes has been gaining importance and prominence. This type of absorber allows vibrating system passive control. In order to better results, it was verified the need to use non-lienar absorbers as these have been shown to receive power more than a small band of vibration frequency of the oscillator. This work considers the vibrating system that consists of a snap-through truss absorber (STTA) coupled to an oscillator under excitation of an electric motor with an eccentricity and limited power, characterizing a non-ideal oscillator (NIO). it is aimed to use the absorver STTA as non-linear absorber and establish the conditions for the motion amplitudes become smaller under certain conditions the mian system. Therefore, this work aims to determine the conditions in which the main system, while it passes through resonance, has attenuated its amplitude of moviment and do not waste energy working in this condition. The main objective of this work is to obtain the conditions under which amplitudes motion of NIO are smaller in the passage through resonance, reducing the Sommerfeld effect. The results obtained in this situation showed that the STTA is effectie to reduce amplitudes motion of NIO before and within region of resonance. In contrast, out of resonance, it was evident that absorber must be removed from the system by imply large amplitudes to the NIO. In addition, a brief study of the performance of STTA on an ideal system was performed. In this situation, the STTA showed significant results in the attenuation of amplitudes motion of the oscillator. Therefore, this work shows that the use of snap-through truss absorber is a good alternative among passive abserbers in attenuation of Sommerfeld effect in non-ideal systems
Mestre

With the shift to learn and consume information through our mobile devices, most academic research is still only presented in long-form text. The Stanford Scholar Initiative has explored the segment of content creation and consumption of academic research through video. However, there has been another popular shift in presenting information from various social media platforms and media outlets in the past few years. Snapchat and Instagram have introduced the concept of tappable “Stories” that have gained popularity in the realm of content consumption. To accelerate the growth of the creation of these research talks, I propose an alternative to video: a tappable Snapchat-like interface. This style is achieved using AMP, Google’s open source project to optimize web experiences on mobile, and particularly the AMP Stories visual medium. My research seeks to explore how the process and quality of consuming the content of academic papers would change if instead of watching videos, users would consume content through Stories on mobile instead. Since this form of content consumption is still largely unresearched in the academic context, I approached this research with a human-centered design process, going through a few iterations to test various prototypes before formulating research questions and designing an experiment. I tested various formats of research consumption through Stories with pilot users, and learned many lessons to iterate from along the way. I created a way to consume research papers in a Stories format, and designed a comparative study to measure the effectiveness of consuming research papers through the Stories medium and the video medium. The results indicate that Stories are a quicker way to consume the same content, and improve the user’s pace of comprehension. Further, the Stories medium provides the user a self-paced method—both temporally and content-wise—to consume technical research topics, and is deemed as a less boring method to do so in comparison to video. While Stories gave the learner a chance to actively participate in consumption by tapping, the video experience is enjoyed because of its reduced effort and addition of an audio component. These findings suggest that the Stories medium may be a promising interface in educational contexts, for distributing scientific content and assisting with active learning.

Introduction: Food insecurity prevalence in Washington County, TN is 14.3% yet 61% of SNAP eligible residents are food insecure. Food insecurity is associated with inadequate consumption of fruits and vegetables, as well as increased risk of chronic disease. Tennessee ranks 47th in the Nation with only 6.7% consuming the recommended fruits and vegetable intake. The Farmacy Pilot Program was developed to encourage increased consumption of produce and to reduce food insecurity among SNAP recipients. Methods: This program provided vouchers to SNAP recipients and their families to buy fresh fruits and vegetables at the farmers market and in the grocery store as a monthly prescription. Participants were recruited from a clinic and two community centers in Washington County. Participants were given $28 - $112 a month depending on household size. Participants were required to attend at least two nutrition classes. Baseline and 6-month follow up assessments were done of food intake patterns among 29 participants, and focus groups were held (n= 11) and a total of 22 interviews conducted. Mixed methods approach was used for analysis: survey data was analyzed on SPSS and thematic analysis conducted for the qualitative data. Results: Major themes that emerged were: decreased cost of produce, increased positive perception of fruits and vegetables, improved perception of their personal health after change in diet, and increased utilization of farmers markets. Barriers identified were cultural and language hurdles, market variability in cost and quality of produce, and transportation. Survey findings included increased intake of green vegetables (t= -2.13, p =.042). Other findings lacked statistical power to detect significance yet were of clinical significance: improvements in frequency of produce consumption, produce variety, and a reduction in food insecurity. Discussion: Providing additional funds targeted on fresh produce can increase food security and increased quantity, frequency and variety of fruits and vegetables consumed. Promoting utilization of farmers market offers a promising avenue for increased consumption of fresh produce and improved social connectedness in the community.

Sandwich structures constitute basic components of advanced supersonic/hypersonic flight and launch vehicles. These advanced flight vehicles operate in hostile environments consisting of high temperature, moisture, and pressure fields. As a result, these structures are exposed to large lateral pressures, large compressive edge loads, and high temperature gradients which can create large stresses and strains within the structure and can produce the instability of the structure. This creates the need for a better understanding of the behavior of these structures under these complex loading conditions. Moreover, a better understanding of the load carrying capacity of sandwich structures constitutes an essential step towards a more rational design and exploitation of these constructions. In order to address these issues, a comprehensive geometrically non-linear theory of doubly curved sandwich structures constructed of anisotropic laminated face sheets with an orthotropic core under various loadings for simply supported edge conditions is developed. The effects of the radii of curvature, initial geometric imperfections, pressure, uniaxial compressive edge loads, biaxial edge loading consisting of compressive/tensile edge loads, and thermal loads will be analyzed. The effect of the structural tailoring of the facesheets upon the load carrying capacity of the structure under these various loading conditions are analyzed. In addition, the movability/immovability of the unloaded edges and the end-shortening are examined. To pursue this study, two different formulations of the theory are developed. One of these formulations is referred to as the mixed formulation, While the second formulation is referred to as the displacement formulation. Several results are presented encompassing buckling, postbuckling, and stress/strain analysis in conjunction with the application of the structural tailoring technique. The great effects of this technique are explored. Moreover, comparisons with the available theoretical and experimental results are presented and good agreements are reported.
Ph. D.

Arch-shaped microelectromechanical systems (MEMS) have been used as mechanical memories, micro-sensors, micro-actuators, and micro-valves. A bi-stable structure, such as an arch, is characterized by a multivalued load deflection curve. Here we study the symmetry breaking, the snap-through instability, and the pull-in instability of bi-stable arch shaped MEMS under steady and transient electric loads. We analyze transient finite electroelastodynamic deformations of perfect electrically conducting clamped-clamped beams and arches suspended over a flat rigid semi-infinite perfect conductor. The coupled nonlinear partial differential equations (PDEs) for mechanical deformations are solved numerically by the finite element method (FEM) and those for the electrical problem by the boundary element method. The coupled nonlinear PDE governing transient deformations of the arch based on the Euler-Bernoulli beam theory is solved numerically using the Galerkin method, mode shapes for a beam as basis functions, and integrated numerically with respect to time. For the static problem, the displacement control and the pseudo-arc length continuation (PALC) methods are used to obtain the bifurcation curve of archâ s deflection versus the electric potential. The displacement control method fails to compute archâ s asymmetric deformations that are found by the PALC method. For the dynamic problem, two distinct mechanisms of the snap-through instability are found. It is shown that critical loads and geometric parameters for instabilities of an arch with and without the consideration of mechanical inertia effects are quite different. A phase diagram between a critical load parameter and the arch height is constructed to delineate different regions of instabilities. The local water slamming refers to the impact of a part of a ship hull on stationary water for a short duration during which high local pressures occur. We simulate slamming impact of rigid and deformable hull bottom panels by using the coupled Lagrangian and Eulerian formulation in the commercial FE software LS-DYNA. The Lagrangian formulation is used to describe planestrain deformations of the wedge and the Eulerian description of motion for deformations of the water. A penalty contact algorithm couples the wedge with the water surface. Damage and delamination induced, respectively, in a fiber reinforced composite panel and a sandwich composite panel and due to hydroelastic pressure are studied.
Ph. D.

A la sinapsis de la unió neuromuscular (NMJ), diverses vies de senyalització coordinen les respostes pre-/postsinàptiques i les cèl·lules glials associades. La relació entre aquestes vies modula les vesícules sinàptiques que regulen la neurotransmissió. A més, la PKC fosforila diverses molècules de l'aparell exocitòtic responsable d'aquesta regulació. Munc18-1 i SNAP-25 són substrats de PKC que juguen un paper clau en la maquinària exocitòtica. A més, la PKC està modulada per l'activitat pre-/postsinàptica al múscul esquelètic. No obstant això, encara es desconeix quines isoformes de PKC regulen aquestes molècules clau en la NMJ. cPKCβI i nPKCƐ es troben exclusivament en el terminal nerviós de la NMJ i estan regulades per l'activitat sinàptica. A més, la contracció muscular a través de BDNF/TrkB té un impacte important en aquestes isoformes. Amb tot, l’objectiu d’aquesta tesi és determinar l’expressió, localització i regulació pels activadors de PKC calci i èsters de forbol (PMA) de Munc18-1 i SNAP-25 i la seva fosforilació en el múscul esquelètic. A més, estudiar si aquestes fosforilacions estan afectades per (1) activitat sinàptica i contracció muscular per se; i (2) nPKCƐ, cPKCβI i la senyalització BDNF/TrkB de manera dependent d’activitat neuromuscular. Els principals resultats, obtinguts mitjançant anàlisi Western blot i microscòpia confocal, mostren que Munc18-1 i SNAP-25 s'expressen i fosforilen en condicions basals en el múscul esquelètic, predominantment a la fracció membrana, essent Munc18-1 localitzat al terminal nerviós. La fosforilació de Munc18-1 i SNAP-25 és modulada per calci, PMA, activitat sinàptica i és promoguda per nPKCƐ. D’altra banda, cPKCβI i la senyalització BDNF/TrkB regulen la fosforilació de Munc18-1 però no la de SNAP-25. Finalment, la contracció muscular regula negativament aquestes proteïnes per assolir un estat basal. En conclusió, aquests resultats proporcionen una visió mecànica de com Munc18-1 i SNAP-25 es regulen per aconseguir l'extraordinària precisió i plasticitat de la neurotransmissió.
En la sinapsis de la unión neuromuscular (NMJ), varias vías de señalización coordinan las respuestas pre-/postsinápticas y las células gliales asociadas. La relación entre estas vías modula las vesículas sinápticas que regulan la neurotransmisión. Además, la PKC, modulada por actividad presináptica y postsináptica en el músculo esquelético, fosforila varias moléculas del aparato exocitótico responsable de esta regulación. Munc18-1 y SNAP-25 son sustratos de PKC que juegan un papel clave en la maquinaria exocitótica. Sin embargo, todavía se desconoce qué isoforma de PKC regula estas moléculas clave en la NMJ. cPKCβI y nPKCƐ se encuentran exclusivamente en el terminal nervioso de la NMJ y están reguladas por actividad sináptica. Además, la contracción muscular a través de BDNF/TrkB tiene un impacto importante en estas isoformas. Así mismo, el objetivo de esta tesis es determinar la expresión, localización y la influencia de los activadores de PKC calcio y ésteres de forbol (PMA) de Munc18-1 y SNAP-25 y su fosforilación en el músculo esquelético. Además, estudiar si dichas fosforilaciones están afectadas por (1) actividad sináptica y contracción muscular per se; y (2) nPKCƐ, cPKCβI y la señalización BDNF/TrkB de manera dependiente de actividad neuromuscular. Los principales resultados, obtenidos mediante análisis Western blot y microscopia confocal, muestran que Munc18-1 y SNAP-25 se expresan y fosforilan en condiciones basales en el músculo esquelético, predominantemente en la fracción membrana, localizándose Munc18-1 en el terminal nervioso. La fosforilación de Munc18-1 y SNAP-25 se modula por calcio, PMA, actividad sináptica y es promovida por nPKCƐ. Por otra parte, cPKCβI y la señalización BDNF/TrkB regulan la fosforilación de Munc18-1 pero no de SNAP-25. Finalmente, la contracción muscular regula negativamente estas proteínas hacia un estado basal. En conclusión, estos resultados proporcionan una visión mecánica de cómo Munc18-1 y SNAP-25 se regulan para lograr la extraordinaria precisión y plasticidad de la neurotransmisión.
At the neuromuscular junction (NMJ) synapse, several signaling pathways coordinate pre-, post-synaptic responses and associated glial cells. The relation between these signaling pathways modulates the pool of synaptic vesicles leading to neurotransmission. Moreover, PKC phosphorylates several molecules of synaptic vesicle exocytotic apparatus responsible to the regulation of neurotransmitter release. Munc18-1 and SNAP-25 are two PKC substrates that play a key role in the exocytotic machinery. In addition, PKC is modulated by presynaptic and postsynaptic activity in skeletal muscles. Nevertheless, it is still unknown which PKC regulates these key molecules in the NMJ. cPKCβI and nPKCƐ are exclusively located at the nerve terminal of the NMJ and are regulated by synaptic activity. In addition, muscle contraction through BDNF/TrkB has an important impact on these PKC isoforms. Therefore, this thesis is aimed to determine the expression, location and regulation by the PKC-activators calcium and phorbol esters (PMA) of Munc18-1 and SNAP-25 and their phosphorylated forms in the skeletal muscle. Also, to study whether Munc18-1 and SNAP-25 phosphorylation are affected by (1) synaptic activity and muscle contraction per se; and (2) nPKCƐ, cPKCβI and BDNF/TrkB signaling in a neuromuscular activity-dependent manner. Main results, obtained by Western blot analysis and confocal microscopy, show that Munc18-1 and SNAP-25 are expressed and phosphorylated in basal conditions in the skeletal muscle, predominantly in the membrane fraction, with Munc18-1 being located at the nerve terminal. Munc18-1 and SNAP-25 phosphorylation are modulated by calcium, PMA, synaptic activity and enhanced by nPKCƐ. Otherwise, cPKCβI and BDNF/TrkB signaling pathway regulates Munc18-1 but not SNAP-25 phosphorylation. Finally, muscle contraction downregulates these proteins to reach a basal state. In conclusion, these results provide a mechanistic insight into how Munc18-1 and SNAP-25 phosphorylation is regulated to achieve the extraordinary precision and plasticity of neurotransmission.

Thesis (Ph.D.)--Aerospace Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Erian Armanios; Committee Member: D. Stefan Dancila; Committee Member: Juan R. Cruz; Committee Member: Massimo Ruzzene; Committee Member: Rami Haj-Ali

碩士
明志科技大學
機電工程研究所
100
The arch shell structure is widely used as the button or switch in the electronic products. Using the non-linear finite element analysis, the snap-through deformation of the arch shell has been studied in this thesis. The large displacement, contact condition and elasto-plastic stress-strain curves are considered in the numerical simulations. In the specified case, the finite element results agree with those of the experiments. From the numerical analysis, the force-displacement curves and click ratios of various cases are obtained. The snap-through behavior of the arch is affected by many conditions such as working temperature, material type and geometry. The JIS SUS 301 EH stainless steel is a good design choice to provide the stable snap-through performance. Finally, this research will provide the information of the button or switch design for engineers.

In this thesis, we present theoretical and experimental investigation into the nonlinear statics and dynamics of clamped-clamped in-plane MEMS arches when excited by an electrostatic force. Theoretically, we first solve the equation of motion using a multi- mode Galarkin Reduced Order Model (ROM). We investigate the static response of the arch experimentally where we show several jumps due to the snap-through instability. Experimentally, a case study of in-plane silicon micromachined arch is studied and its mechanical behavior is measured using optical techniques. We develop an algorithm to extract various parameters that are needed to model the arch, such as the induced axial force, the modulus of elasticity, and the initially induced initial rise. After that, we excite the arch by a DC electrostatic force superimposed to an AC harmonic load. A softening spring behavior is observed when the excitation is close to the first resonance frequency due to the quadratic nonlinearity coming from the arch geometry and the electrostatic force. Also, a hardening spring behavior is observed when the excitation is close to the third (second symmetric) resonance frequency due to the cubic nonlinearity coming from mid-plane stretching. Then, we excite the arch by an electric load of two AC frequency components, where we report a combination resonance of the summed type. Agreement is reported among the theoretical and experimental work.

The primary focus of this dissertation is the characterization of snap-through buckling of discrete and continuous systems. Snap-through buckling occurs as the consequence of two factors, first the destabilization, or more often the disappearance of, an equilibrium position under the change of a system parameter, and second the existence of another stable equilibrium configuration at a remote location in state space. In this sense snap-through buckling is a global dynamic transition as the result of a local static instability.

In order to better understand the static instabilities that lead to snap-through buckling, the behavior of mechanical systems in the vicinity of various local bifurcations is first investigated. Oscillators with saddle-node, pitchfork, and transcritical bifurcations are shown analytically to exhibit several interesting characteristics, particularly in relation to the system damping ratio. A simple mechanical oscillator with a transcritical bifurcation is used to experimentally verify the analytical results. The transcritical bifurcation was selected since it may be used to represent generic bifurcation behavior. It is shown that the damping ratio may be used to predict changes in stability with respect to changing system parameters.

Another useful indicator of snap-through is the presence of chaos in the dynamic response of a system. Chaos is usually associated snap-through, as in many systems large amplitude responses are typically necessary to sufficiently engage the nonlinearities that induce chaos. Thus, a pragmatic approach for identifying chaos in experimental (and hence noisy) systems is also developed. The method is applied to multiple experimental systems showing good agreement with identification via Lyapunov exponents.

Under dynamic loading, systems with the requisite condition for snap-through buckling, that is co-existing equilibria, typically exhibit either small amplitude response about a single equilibrium configuration, or large amplitude response that transits between the static equilibria. Dynamic snap-through is the name given to the large amplitude response, which, in the context of structural systems, is obviously undesirable. This phenomenon is investigated using experimental, numerical, and analytical means and the boundaries separating safe (non-snap-through) from unsafe (snap-through) dynamic response in forcing parameter space are obtained for both a discrete and a continuous arch. Arches present an ideal avenue for the investigation of snap-through as they typically have multiple, often tunable, stable and unstable equilibria. They also have many direct applications in both civil engineering, where arches are a canonical structural element, and mechanical engineering, where arches may be used to approximate the behavior of curved plates and panels such as those used on aircraft.

Dissertation

Recent proposed rafter slenderness limits, to prevent snap-through of plastically designed pitched-roof portal frames, incorporate the elastic snap-through buckling load of such frames. It has been suggested that the elastic snap-through buckling load used in the proposals is over-estimated making these slenderness limits unconservative. This is supported by a more rigorous elastic analysis. To test the proposals, model frames lying on or close to the slenderness limits were tested to failure in the laboratory. Frame dimensions were chosen so that the frames were only susceptible to snap-through instability. Failure loads far lower than the expected plastic collapse loads were measured, showing that the elastic snap-through buckling load is over -estimated . Since plastic analysis is easily applied to portal frames, these slenderness limits are best replaced by a similar limit incorporating a more accurate elastic snap-through buckling load. A new limit is outlined which must still be tested by further research

碩士
國立臺灣大學
機械工程學研究所
91
In this paper we study the dynamic behavior of a shallow arch under a moving load system containing two masses, a dashpot, and a suspension spring. This paper is an extension of a previous publication [22] in which the arch is loaded by a moving point force. The emphasis of the paper is placed on finding out how the inertia effect of the load system affects the dynamic response of the arch. It is assumed that the masses in the load system are under the action of gravity while the gravity on the arch itself is neglected. It is found that the point-force model is a good approximation only when the arch is slender and the moving speed of the load system is low. The boundary of a dangerous speed zone is defined based on the comparison of the total energy gained by the arch and an energy barrier. It is observed that the suspension model predicts a considerably different dangerous speed zone from the point-force model, especially in the high speed range.

碩士
國立臺灣大學
機械工程學研究所
95
In this paper we investigate, both numerically and experimentally, both the static and dynamic case, the end couple is applied in the form of a step function with infinite duration. Attention is focused on the prediction of the snap-through buckling loads, both static and dynamic. The relations between the critical load with different initial heights are presented. In the experiment we examine the arch when one end and both ends are affected by moment increased slowly, the change of the height of mid point of the arch, later we amount examined the proportion that the amplitude decayed when the arch mid point was beaten, used for estimating the arch damping coefficient, and quantity examine the arch on strength and receive natural frequency of situation of strength, we amount finally examine the arch happen person who break at the song of frustrating until one end and both ends affect by steps form moment of the step, the behavior that the mid point highly changes over time, and the data won of the testing amount, with taking the place of and entering estimating that contrast each other in result that will be received of simulation of counting value of damping coefficient.

abstract: In the delivery of a public service, meeting the needs of its users through cocreation has generated considerable research. Service users are encouraged to engage with public services through dialogue, sustained interaction, and equal partnership, wherein the role of the user changes from passive to active. As the relationship between service provider and service user evolves, researchers have sought to explain how resources, time, accessibility, and bandwidth may affect such relationships, specifically concerning the economically disadvantaged. While many researchers have focused on the logistical barriers that inhibit cocreation among the economically disadvantaged presented by such factors as cost and transportation, limited research has examined the relationship between the service provider and economically disadvantaged service user. Combining previous research, this study examines what economically disadvantaged service users actually do when they cocreate value with a public service by conducting 12 in-depth interviews with participants of SNAP-Ed, nutrition education for persons eligible for government assistance. The study's findings suggest that cocreation exists through relational characteristics of collaboration, isolation, acceptance, connection, and guidance that help in the development and maintenance of relationships, and that a relationship between service provider and user could be further typified by equality. This finding suggests that equality is an independent construct not necessary in the process of cocreation--a departure from previous research--but rather a way to approach the service provider/user relationship. This study is intended as a step toward examining cocreation through the development of organization-public relationships.
Dissertation/Thesis
Doctoral Dissertation Journalism and Mass Communication 2014

碩士
國立臺灣大學
機械工程學研究所
91
In this paper we study the effects of elastic foundation on the static and dynamic snap-through of a shallow arch under a point load traveling at a constant speed. The deformation of the arch is expressed in a Fourier series. For static analysis when the moving speed of the point load is almost zero, the first four modes in the expansion are sufficient in predicting the equilibrium positions and the critical loads. Unlike the case without elastic foundation, static snap-through can occur even when the arch is in another stable position before the point load moves onto the arch. In the dynamic case when the moving speed of the point load is significant, the numerical simulation of the response does not converge well, especially long after the point load leaves the arch. However, the total energy of the arch converges quite well when only the first eight modes are used in the Fourier series. This observation allows us to establish a sufficient condition against dynamic snap-though, although we are unable to predict precisely, with finite number of modes in the series, at what time it will occur when this sufficient condition is not fulfilled.